With the advent of the benzene-toluene-C.sub.8 aromatics fraction (known and hereinafter referred to at BTX) as the principal raw material in the manufacture of petrochemicals, outstripping ethylene in this regard, and the increased demand for aromatics as a component in gasoline, to increase its octane rating and thus reduce or eliminate the need for lead, which has been under fire as a pollutant, aromatics processes availed of in the past have come under close scrutiny with an eye toward improving process economics, which can be translated into, among other things, the use of less apparatus and decreased heat requirements.
The recovery of aromatic hydrocarbons by selective extraction and distillation of hydrocarbon mixtures containing relatively polar compounds (aromatic and olefinic groups) and relatively less polar compounds (paraffinic and napthenic groups) is well known. A wide variety of techniques can be used. The following are typical prior art techniques:
(1) In the recovery of benzene, toluene, and C.sub.8 aromatics from petroleum fractions, tetraethylene glycol is used as the selective extraction solvent. The BTX is steam distilled from the solvent which remains as bottoms and is recycled to the extraction step; PA1 (2) In the recovery of benzene, toluene, and C.sub.8 aromatics from petroleum fractions, sulfolane-water mixtures (2-4 percent water by weight) are used as the selective extraction solvent. The nonaromatics are separated from the rich solvent in a stripper at pressures that are slightly higher than atmospheric pressure. These nonaromatics are sent back to the extraction zone as reflux. The BTX is separated from the solvent in a recovery column at about 450 mm Hg. The solvent remains as bottoms and is recycled to the extraction step; and PA1 (3) Kerosenes can be treated with liquid SO.sub.2 and this solvent is then distilled from the extracted aromatics.
Further examples include, Poffenberger's U.S. Pat. No. 2,711,433 which discloses an improved process for the selective solvent extraction and recovery of aromatic hydrocarbons from mixtures with non-aromatic hydrocarbons. The particular method provides for the separation and recovery of individual aromatic hydrocarbons of industrial grade or better without the need for chemical treatment. This particular patent claims heat economy, however a very complicated five column extraction-distillation process is used.
Fenske and McCormick in U.S. Pat. No. 2,909,576 disclose a method for the separation of benzene from virgin or cracked naphthas with the use of NH.sub.3 as the solvent. The process operates under the theory that azeotropes of nonaromatic hydrocarbons with NH.sub.3 are formed that can be separated from aromatic hydrocarbons. Though this patent claims heat recovery through the use of compressors, the pressures used are excessively high, and therefore require more expensive equipment to handle these higher pressures. Furthermore, the aromatic product obtained has low purity.
Uitti and Okuma in U.S. Pat. No. 3,367,846 claim a process for the separation of aromatic hydrocarbon mixtures of benzene, toluene and xylene utilizing fractional distillation. Low pressure columns are used in order to recover heat. The heat of condensation of the top vapors of the xylene column are employed to generate steam which is then used to operate the benzene column. This process however, proves useful only in the separation of aromatics hydrocarbons from each other.
Though these separation techniques are or have been widely used in industry, they demand a great deal of heat for the distillation steps. It is, therefore, very desirable to reduce the heat load costs in such processes.